The invention relates generally to a switch mode power supply and more specifically to preventing the RF noise generated from a switch mode power supply from interfering with the reception of a radio signal in a nearby radio receiver.
The operation of a switch mode power supply generates noise impulses at frequencies which extend into the frequencies of AM broadcast frequencies. When the noise signal is received by a radio receiver, the noise signal typically demodulates into an undesirable tone in the audible range of a human ear (20 Hz-22 KHz). One method to reduce the received noise is to locate the radio receiver away from the noise source. However, in automotive applications and, more specifically, in an automobile instrument panel, the distance a radio receiver can be separated from a switch mode power supply utilized in the instrument panel is limited.
In prior art applications, the operating frequency of a switch mode power supply is controlled by timing circuits such as RC circuits. The operating frequency of such circuits tend to drift over the temperature range typically encountered in automobiles. Thus, a time variable RF noise component may be picked up by an AM receiver.
It is therefore desirable to provide a switch mode power supply operating in close proximity to a radio receiver without producing substantial audible noise in the demodulated output of the radio receiver.